exerciselist: Support sphinx<4.0, which uses master_doc instead of root_doc

[rkdarst]

• Sphinx 4.0 renamed master_doc to root_doc, so exerciselist didn't work on that earlier version.
• The Github Action actually does try to test for earlier versions, but it seems like it does n't successfully install the older Sphinx versions for the tests. Re-adjust the action to install everything at once, in hopes that this works. A local test implies that it might.
• Closes: #84
• Review:
  • does CI pass?
  • Check the action output "List dependency version" log and verify that it's testing older sphinx versions.

[rkdarst]

This seems to work.